One-stage process for zinc phosphating

ABSTRACT

The present invention relates to a process for anticorrosion pretreatment of multiple components in series, each component in the series at least partly comprises metal surfaces of zinc, iron and/or aluminum and undergoes a zinc phosphating step in which the component is contacted with an acidic aqueous composition containing an amount of an activating aid sufficient to ensure a layer weight below 5.5 g/m2 on a cleaned, untreated hot-dip galvanized steel surface (Z), wherein the activating aid is based on a water-dispersed particulate constituent at least partly selected from hopeite, phosphophyllite, scholzite and/or hureaulite, and at least one polymeric organic compound; and further relates to acidic aqueous zinc phosphating compositions obtainable by adding a particular amount of a colloidal aqueous solution containing the dispersed particulate constituent to an acidic aqueous composition containing zinc ions, phosphate ions and free fluoride.

The present invention relates to a process for anticorrosion pretreatment of a multitude of components in series, in which each component in the series at least partly has surfaces of the metals zinc, iron and/or aluminum and undergoes a process step for zinc phosphating and in the process is brought into contact with an acidic aqueous composition to which there has been added such an amount of an activating aid which is sufficient to ensure a layer weight below 5.5 g/m² on a hot-dip galvanized steel surface (Z) that has merely been cleaned and is otherwise untreated. The activating aid is based on a water-dispersed particulate constituent at least partly selected from hopeite, phosphophyllite, scholzite and/or hureaulite, and at least one polymeric organic compound. According to the invention an acidic aqueous composition for zinc phosphating is also comprised, which can be obtained by adding a particular amount of a colloidal aqueous solution containing the dispersed particulate constituent to an acidic aqueous composition containing zinc ions, phosphate ions and free fluoride.

Layer-forming phosphating is a process for applying crystalline anticorrosion coatings to metal surfaces, in particular to materials of the metals iron, zinc and aluminum, which has been used for decades and has been studied in depth. Zinc phosphating, which is particularly well established for corrosion protection, is carried out in a layer thickness of a few micrometers and is based on corrosive pickling of the metal material in an acidic aqueous composition containing zinc ions and phosphates. In the course of the pickling, an alkaline diffusion layer forms on the metal surface, which layer extends into the interior of the solution and within which sparingly soluble crystallites form, which crystallites precipitate directly at the interface with the metal material and continue to grow there. To support the pickling reaction on materials of the metal aluminum and to mask the bath poison aluminum, which in dissolved form disturbs the layer formation on materials of the metal, water-soluble compounds that are a source of fluoride ions are often added.

The zinc phosphating is adjusted as standard in such a way that homogeneous, closed and compact crystalline coatings are achieved on the surfaces of the metals iron, zinc and aluminum. Otherwise, good corrosion protection and a good coating base cannot be achieved. Homogeneous, closed coatings in zinc phosphating are usually reliably achieved from a layer weight of 2 g/m². Depending on the metal surface to be phosphated, the pickling described above and the concentration of the active components in the zinc phosphating stage have to be adjusted accordingly in order to ensure correspondingly high layer weights on the surfaces of the metals iron or steel, zinc and aluminum.

Another property of zinc phosphating that is important for corrosion protection and coating adhesion, in particular for good electrocoating properties, is that the deposition process is self-limiting, i.e. the dissolution of the phosphate layer, which takes place at the acidic pH value of zinc phosphating, is in a steady-state equilibrium with the growth or continued growth of the phosphate crystallites, and the layer weight therefore no longer increases, which would be an indication of the growth of a layer coating that is crystalline but porous and therefore not compactly crystalline. In the technical zinc phosphating process, this means that, in the case of a treatment time of usually approximately 20 seconds to 5 minutes, which makes sense in terms of plant technology and cost-efficiency, in the zinc phosphating wet-chemical process step, the formation of the homogeneous, closed and crystalline zinc phosphate coating has to be completed and the self-limiting thickness of the coating is already ideally reached. This is technically ensured by the fact that coatings grow with the highest possible number density of phosphate crystallites, so that the layer formation in turn reaches the self-limiting range and thus a predetermined limit layer thickness with the lowest possible layer weights.

In order to achieve such homogeneous, closed coatings with a high degree of compactness or a high number density of phosphate crystallites, in the prior art zinc phosphating is always initiated with activation of the metal surfaces of the component to be phosphated. Activation is usually a wet-chemical process step that conventionally takes place by means of contact with colloidal, aqueous solutions of phosphates (“activation stage”), which, insofar as they are immobilized on the metal surface, are used in the subsequent phosphating as a growth nucleus for the formation of the crystalline coating within the alkaline diffusion layer, so that a high number density of growing crystallites is achieved and thus a compact crystalline zinc phosphate layer is generated in turn, which layer has excellent corrosion protection and, due to its high electrical charge transfer resistance, also has excellent electrocoating properties.

Suitable dispersions in this case are colloidal, mostly neutral to alkaline aqueous compositions based on phosphate crystallites, which have only small crystallographic deviations in their crystal structure from the type of zinc phosphate layer to be deposited. In this context, WO 98/39498 A1 teaches in particular bivalent and trivalent phosphates of the metals Zn, Fe, Mn, Ni, Co, Ca and Al, it being technically preferred for phosphates of the metal zinc to be used for activation for subsequent zinc phosphating.

An activation stage based on dispersions of bivalent and trivalent phosphates requires a high level of process control in order to keep the activation performance constantly at an optimal level, in particular when treating a series of metal components. To ensure the process is sufficiently robust, foreign ions carried over from previous treatment baths or aging processes in the colloidal aqueous solution must not lead to the activation performance deteriorating. A deterioration is initially noticeable in increasing layer weights in the subsequent phosphating and ultimately leads to the formation of defective, inhomogeneous or less compact phosphate layers. Overall, the layer-forming zinc phosphating with upstream activation is therefore a multi-stage process that is technically complex to control and has hitherto been carried out in a resource-intensive manner, both with regard to the process chemicals and the energy to be expended.

WO 2019/238573 A1 addresses a resource-saving process for zinc phosphating and indirectly also a reduction in the complexity of the multi-stage process by providing a particularly effective activation based on specifically dispersed bivalent and trivalent phosphates, which activation provides a colloidal, aqueous solution that is based on bivalent and trivalent phosphates and is excellently stabilized against sedimentation, and also makes homogeneous, closed and very compact zinc phosphate coatings possible with a relatively low particulate content in the activation stage, so that the material requirement due to the formation of layers in the zinc phosphating is also reduced.

However, there is still a need to optimize the pretreatment line for zinc phosphating, including the activation stage and phosphating stage, such that the overall process can be carried out in a less resource-intensive manner, ideally with a simultaneously simplified procedure. However, a resource-saving overall process must not be at the expense of the properties of the zinc phosphating, which must be provided as a homogeneous, closed and compact crystalline coating with high electrical charge transfer resistance in order to make good protection against corrosion and correspondingly good coverage of the coating in a subsequent electrocoating possible. In particular, this must always be ensured in the most common application, specifically the series treatment of components.

Surprisingly, this complex requirement profile can be met by maintaining the activation performance of a pretreatment line for zinc phosphating by metering an activating aid into the zinc phosphating wet-chemical treatment stage. This makes it possible to at least partially or even entirely dispense with an activation stage upstream of the zinc phosphating wet-chemical treatment stage, and in this way to conduct the overall process of zinc phosphating in a less material-intensive and energy-intensive manner and to reduce the technical complexity in the form of the separate activation stage, which has previously been absolutely necessary in the prior art.

The present invention therefore relates to a process for anticorrosion pretreatment of a multitude of components in series, in which each component of the series at least partly has surfaces of the metals zinc, iron and/or aluminum and undergoes a process step for zinc phosphating and in the process is brought into contact with an acidic aqueous composition, the acidic aqueous composition containing

(A) 5-50 g/kg of phosphates dissolved in water, calculated as PO₄, (B) 0.3-3 g/kg of zinc ions, and (C) free fluoride, and having a free acid in points greater than zero, in the process step for zinc phosphating an activating aid (D) being added continuously or discontinuously to the acidic aqueous composition in an amount that, under the selected conditions of the zinc phosphating process step, is sufficient to maintain the property of the acidic aqueous composition of depositing a zinc phosphate layer having a layer weight of less than 5.5 g/m², preferably less than 5.0 g/m², particularly preferably less than 4.5 g/m², on a hot-dip galvanized steel surface (Z), the activating aid (D) containing a particulate constituent (a) in water-dispersed form, which constituent comprises

-   -   at least one particulate inorganic compound (a1) composed of         phosphates of polyvalent metal cations at least partly selected         from hopeite, phosphophyllite, scholzite and/or hureaulite,     -   and at least one polymeric organic compound (a2).

A pretreatment in series is when the components of the series each undergo a process step for zinc phosphating according to the process according to the invention and for this purpose are brought into contact with at least one bath liquid for zinc phosphating that is provided in a system tank, the individual components being brought into contact one after the other and thus at different times. The system tank is in this case the container in which the acidic aqueous composition is located for the purpose of zinc phosphating by way of a wet-chemical pretreatment. The components can be brought into contact with the bath liquid of the system tank inside the system tank, for example by immersion, or outside the system tank, for example by spraying on the bath solution stored in the system tank.

The components treated according to the present invention can be three-dimensional structures of any shape and design that originate from a manufacturing process, in particular also including semi-finished products such as strips, sheets, rods, pipes, etc., and composite structures assembled from said semi-finished products, the semi-finished products preferably being interconnected by means of adhesion, welding and/or flanging to form a composite structure.

In the context of the process according to the invention, a component has at least one surface made of the metals zinc, iron and/or aluminum if more than 50 at. % of the metal structure on this surface, up to a material penetration depth of at least one micrometer, is composed of one of the aforementioned metals. This regularly applies to components made of metal materials, more than 50 at. % of the metal materials, as homogeneous materials, being composed of zinc, iron or aluminum, but also applies to components comprising materials that are provided with metal coatings, such as electrolytically galvanized or hot-dip galvanized strip steel, which can also be alloyed with iron (ZF), aluminum (ZA) and/or magnesium (ZM).

The inventive property of the acidic aqueous composition for zinc phosphating on hot-dip galvanized steel surfaces (Z) of bringing about the growth of a zinc phosphate layer having a layer weight below 5.5 g/m², preferably below 5.0 g/m², particularly preferably below 4.5 g/m², (hereinafter referred to as “phosphating quality”) is to be checked on substrates that have merely been cleaned and degreased (Z) and are not subjected to any further wet-chemical pretreatment step before being brought into contact with the acidic aqueous composition of the process according to the invention. In order to check the phosphating quality of the acidic aqueous composition, hot-dip galvanized steel (Z) is thus first cleaned using an alkaline cleaning agent prepared as 2 wt. % Bonderite® C-AK 1565 A and 0.2 wt. % Bonderite® C-AD 1270 in deionized water (κ<1 μScm⁻¹) at pH 11.0 and 55° C. for 5 minutes by immersion. The substrates cleaned and degreased (Z) in this way are rinsed with deionized water (κ<1 μScm⁻¹) at room temperature and are then supplied to the zinc phosphating process step according to the selected process according to the invention. According to the selected process according to the invention means at a relevant identical temperature, application duration and bath circulation and using the acidic aqueous composition for which the phosphating quality specified according to the invention is to apply, i.e. the resulting target layer weights on hot-dip galvanized steel (Z) are to be below 5.5 g/m², preferably below 5.0 g/m², particularly preferably below 4.5 g/m². The phosphating quality can therefore be determined in the current process according to the invention by cleaned and degreased sheets of hot-dip galvanized steel (Z) also being introduced, together with the components of the series, for the zinc phosphating process step, and the layer weight of zinc phosphate on the sheets, and thus the phosphating quality of the acidic aqueous composition for zinc phosphating, then being determined in the process according to the invention. The cleaned and degreased sheets of hot-dip galvanized steel (Z), in their function as test sheets for determining phosphating quality, are preferably rigidly connected to the component or the conveying frame in order to ensure that the flow conditions during transport of the component together with the conveying frame through the phosphating bath are reproduced as similarly as possible for the test sheet. For this purpose, the test sheets should ideally be connected to the component or the conveying frame in such a way that the transport of a test sheet together with the component and the conveying frame, compared to the transport of the component and the conveying frame without such a test sheet, has no influence on the flow conditions that is to be considered, and that the flow conditions are substantially identical in both cases and thus substantially correspond to the flow conditions of at least a portion of the components of the series. This can be achieved, for example, by adapting the size and/or the shape of the test sheet to the size and shape of the component and/or of the conveying frame which is arranged adjacent to the test sheet in each case. It is conceivable in this case, in particular when a test sheet is arranged on an outer surface portion of the component or the conveying frame, for the dimensions of the test component to correspondingly be smaller than those of said surface portion, for example in order to prevent the test component from protruding beyond the surface portion. Alternatively or additionally, the test component may follow a curvature or other planar deviation of the surface portion or the conveying frame. It has proven to be particularly expedient to select a sheet portion that is sufficiently small compared to the size of a suitable outer surface of the component, with an outer surface being particularly suitable if it is located at a location that has a particularly low curvature or at the location that has the lowest curvature of the component, and the test sheet metal then being mounted substantially in parallel so as to be spaced apart along the surface normal of such an outer surface.

It is additionally preferred for the phosphating quality that, if the contact is extended by one minute, the layer weight on hot-dip galvanized steel (Z) increases by no more than 0.2 g/m², and thus the layer formation under the selected conditions is already in the range of self-limitation, so that the property of the acidic aqueous composition for zinc phosphating is guaranteed to produce compact, crystalline zinc phosphate layers in the process according to the invention. Accordingly, it is preferred that in the process step for zinc phosphating, activating aid (D) is added in an amount that, under the selected conditions of the zinc phosphating process step in the process according to the invention, is sufficient to maintain the property of the acidic aqueous composition of depositing a zinc phosphate layer having a layer weight of less than 5.5 g/m², preferably less than 5.0 g/m², particularly preferably less than 4.5 g/m², on a hot-dip galvanized steel surface (Z), the layer weight achieved under the selected conditions of the zinc phosphating process step in the process according to the invention increasing by no more than 0.2 g/m² if the contact time with the acidic aqueous composition is extended by 60 seconds.

Usually, the phosphating quality is determined and monitored in the process according to the invention by hot-dip galvanized steel (Z), which has been cleaned and degreased as described above, also undergoing the zinc phosphating process step at regular intervals during the series treatment and then being subjected to a layer weight determination. Insofar as the phosphating quality of the acidic aqueous composition is ensured by metering in the activating agent (D), homogeneous, closed and compact crystalline zinc phosphate coatings are deposited on the components having surfaces of the metals zinc, iron and aluminum in the usual treatment times of 20 seconds to 5 minutes.

The layer weight of zinc phosphate is determined within the scope of the present invention by removing the zinc phosphate layer using aqueous 5 wt. % CrO₃ as a pickling solution that is brought into contact with a defined area of the phosphated material or component at 25° C. for 5 min immediately following the zinc phosphating and washing with deionized water (κ<1 μScm⁻¹), and subsequently determining the phosphorus content in the same pickling solution by means of ICP-OES. The layer weight of zinc phosphate can be found by multiplying the amount of phosphorus relative to surface area by a factor of 6.23.

In the process according to the invention, the activating aid (D) is added to the acidic aqueous composition for zinc phosphating for the purpose of maintaining the phosphating quality in the process step for zinc phosphating. In order to maintain the phosphating quality in the series treatment process, the addition can take place by means of continuous or discontinuous metering into the system tank. Continuous metering is preferred if the pretreatment of the components in series directly follows one another and the decrease in the phosphating quality over time can be determined, so that a quantity of the activating agent can in turn be continuously metered in over time. This process has the advantage that, after the start-up of the pretreatment line and the determination of the material flows for the metering of the activating aid and other active components, the phosphating quality does not have to be checked further as long as the series treatment remains unchanged in terms of timing and properties of the components to be treated and the treatment parameters in the zinc phosphating process step. However, if a constant mode of operation in the series treatment cannot be ensured or is not desirable due to the system, discontinuous metering of the activating aid is advantageous and may even be advisable. In this case, the phosphating quality of the acidic aqueous composition is preferably monitored continuously or at defined time intervals, and a specified amount of the activating aid is then metered in if the layer weight on hot-dip galvanized steel (Z) reaches a certain value below 5.5 g/m², preferably below 5.0 g/m², particularly preferably below 4.5 g/m². The continuous or quasi-continuous determination of the phosphating quality, which takes place at defined time intervals, can also be carried out using proxy data that correlate with the actual zinc phosphate layer weight. The non-destructive determination of the layer thickness, for example using the eddy current process or even contact-free optical determination methods such as ellipsometry or spectral reflectivity measurement, provides suitable proxy data for the layer weight of zinc phosphate, which data can be reliably measured on the components in a pretreatment line and can be correlated with the actual layer weight on hot-dip galvanized parts steel (Z). The crystallite size and thus the determination of the roughness by means of optical profilometry can also provide proxy data for the layer weight, since a higher layer weight on hot-dip galvanized steel (Z) is associated with a low number density of crystallites, which, however, are relatively larger, so that the roughness increases with the layer weight.

It has been found that the phosphating quality is already adequate in most cases if the activating aid (D) is metered in continuously or discontinuously in such an amount that is suitable for maintaining a steady-state amount of preferably at least 0.001 g/kg, particularly preferably at least 0.005 g/kg, more particularly preferably at least 0.01 g/kg, of particulate constituent (a) in the acidic aqueous composition during the pretreatment of the components in series. This applies in particular to the contacting of the acidic aqueous composition by spraying, whereas in the case of dip application, a steady-state amount of preferably at least 0.002 g/kg, particularly preferably 0.01 g/kg and more particularly preferably 0.02 g/kg of particulate constituent (a) should be contained in the acidic aqueous composition for zinc phosphating.

The present invention thus surprisingly shows that by metering an activating aid, as is known in the prior art and described, for example, in WO 98/39498 A1, directly into acidic aqueous treatment solution for zinc phosphating, activation of the metal surfaces can take place, so that homogeneous, closed and compact crystalline zinc phosphate coatings having a high electrical charge transfer resistance grow on the metal surfaces. The present invention makes use of this effect by focusing on maintaining the phosphating quality in the series treatment of components by metering the activating aid into the acidic aqueous composition for zinc phosphating. For the desired phosphating quality, it is in this case possible to switch to solely metering the activating aid, without the components in the series having to go through a wet-chemical activation stage, e.g. based on an activating aid (D), before the zinc phosphating process step. This can save a complete process step, including the necessary bath maintenance, circulation, temperature management and chemical additivation, e.g. using water-soluble condensed phosphates, so that an extremely resource-saving and economical operation of a pretreatment line for zinc phosphating is possible for the first time.

In a preferred embodiment of the process according to the invention, before being brought into contact with the acidic aqueous composition in the zinc phosphating process step, the components of the series are therefore not brought into contact with a colloidal, aqueous activation solution containing, in the particulate constituent, hopeite, phosphophyllite, scholzite and/or hureaulite, preferably phosphates of polyvalent metal cations, or sparingly soluble salts of the element Ti. Before being brought into contact with the acidic aqueous composition in the zinc phosphating process step, the components of the series are particularly preferably not brought into contact with a colloidal, aqueous solution for activating the surfaces of the components for zinc phosphating, and the components of the series very particularly preferably do not go through an activation stage for activating the surfaces of the components for zinc phosphating before being brought into contact.

However, it is usually not possible to dispense with a cleaning and degreasing stage as a process step upstream of the zinc phosphating. In order to achieve reproducible layer coatings that are as uniform as possible, in a preferred embodiment of the process according to the invention, at least the metal surfaces of the components are cleaned and if necessary degreased in a cleaning stage before the zinc phosphating process step. The cleaning is preferably carried out by contact with an aqueous, preferably neutral or alkaline cleaning agent, the zinc phosphating process step preferably immediately following the cleaning stage, with or without an intermediate rinsing step. The alkaline cleaning is characterized by the fact that the metal surfaces, in particular the surfaces that contain metal aluminum, whether as a material or as an alloy component of hot-dip galvanized steel, are pickled, which leads to an additional standardization of the metal surfaces and is therefore advantageous for the growth of homogeneous zinc phosphate coatings. The cleaning stage is preferably does not take place by means of contact with an aqueous, preferably neutral or alkaline cleaning agent containing a particulate constituent comprising hopeite, phosphophyllite, scholzite and/or hureaulite or sparingly soluble salts of the element Ti, since, as explained above, any activation of the metal surfaces before the zinc phosphating can be dispensed with according to the invention. A rinsing step after the cleaning, as already mentioned, is optional and, in the context of the present invention, is used exclusively for the complete or partial removal of soluble residues, particles and active components that are carried over from a previous wet-chemical treatment step, in this case the cleaning and degreasing stage, by adhering to the component, from the component to be treated, without active components based on metal or semi-metal elements, which are already consumed merely by the metal surfaces of the component being brought into contact with the rinsing liquid, being contained in the rinsing liquid itself. For example, the rinsing liquid can simply be city water or deionized water or, if necessary, can also be a rinsing liquid that contains surface-active compounds to improve the wettability by means of the rinsing liquid.

Since the phosphating quality in the process according to the invention on hot-dip galvanized steel is technically optimized, processes are naturally also preferred according to the invention in which the components of the series at least partly have surfaces of the metal zinc, which are in particular selected from hot-dip galvanized steel surfaces. In principle, the phosphating quality of the acidic aqueous composition, which is maintained in the process according to the invention by adding the activating aid (D), is such that components that are manufactured in a multi-metal construction, such as automobile bodies, can also be zinc-phosphated with very good properties and very homogeneous, closed and compact zinc phosphate coatings can also be obtained on iron and aluminum surfaces. In the process according to the invention, it is therefore preferred for the components in the series to also have surfaces of the metal iron or, specifically for lightweight construction in car body manufacturing, additional aluminum. In a particularly preferred embodiment, specifically in car body manufacturing, the components have surfaces of the metals zinc, iron and aluminum next to one another.

In the process according to the invention, it is preferred for the components to be brought into contact with the acidic aqueous composition for at least a period of time that is sufficient to deposit a layer weight of at least 1.0 g/m² on the zinc surfaces, since it is then ensured that a sufficiently homogeneous, closed zinc phosphate coating is formed on all metal surfaces of the components selected from zinc, iron and aluminum. Accordingly, preference is given to a process according to the invention in which a zinc phosphate layer having a layer weight of at least 1.0 g/m², preferably at least 1.5 g/m², is deposited on the zinc surfaces. Since the phosphating quality of the acidic aqueous composition for zinc phosphating is maintained as a control variable in the process according to the invention and the acidic aqueous composition inherently has a sufficient activation performance, it is also always ensured that the zinc surfaces of the component have a homogeneous, closed and compact crystalline zinc phosphate layer, the layer thickness of which is in the self-limiting range, so that, according to the invention, the layer weight of the zinc phosphate layer on the zinc surfaces of the component is preferably below 5.5 g/m², more preferably below 5.0 g/m² and particularly preferably below 4.5 g/m².

Activating aids (D) that can be used according to the invention, i.e. that maintain the phosphating quality during metering into the acidic aqueous composition of the zinc phosphating, are aqueous dispersions and thus contain a particulate constituent (a) in water-dispersed form which comprises at least one particulate inorganic compound (a1) composed of phosphates of polyvalent metal cations at least partly selected from hopeite, phosphophyllite, scholzite and/or hureaulite, and at least one polymeric organic compound (a2).

The use of polyvalent metal cations in the form of phosphates is responsible for the good activation performance or suitability of the activating aid (D) for maintaining the phosphating quality of the acidic aqueous composition for zinc phosphating, and said phosphates should therefore be contained in the activating aid (D) in a sufficiently high proportion in the dispersed particulate constituent (a). Accordingly, the proportion of phosphates contained in the at least one particulate inorganic compound (a1), based on the dispersed particulate constituent (a) in the activating aid, is preferably at least 25 wt. %, particularly preferably at least 35 wt. %, more particularly preferably at least 40 wt. %, very particularly preferably at least 45 wt. %. The dispersed particulate constituent (a) of the activating aid (D) is the solids content that remains after drying the retentate of an ultrafiltration of a defined partial volume of the activating aid (D) with a nominal cutoff limit of 10 kD (NMWC: nominal molecular weight cutoff). The ultrafiltration is carried out by adding deionized water (κ<1 μScm⁻¹) until a conductivity of below 10 μScm⁻¹ is measured in the filtrate. The inorganic particulate constituent in the activating aid (D) is, in turn, that which remains when the particulate constituent (a) obtained from the drying of the ultrafiltration retentate is pyrolyzed in a reaction furnace by supplying a CO₂-free oxygen flow at 900° C. without admixture of catalysts or other additives until an infrared sensor provides a signal identical to the CO₂-free carrier gas (blank value) in the outlet of the reaction furnace. The phosphates contained in the inorganic particulate constituent are determined as phosphorus content by means of atomic emission spectrometry (ICP-OES) after acid digestion of the constituent with aqueous 10 wt. % HNO₃ solution at 25° C. for 15 min, directly from the acid digestion.

The active components of the activating aid (D), which, as soon as they are metered into the acidic aqueous composition for zinc phosphating in a sufficient amount, promote the formation of a homogenous, closed and compact phosphate coating on the metal surfaces and in this sense activate the metal surfaces, as already mentioned, are composed primarily of phosphates, which in turn are at least partly selected from hopeite, phosphophyllite, scholzite and/or hureaulite, preferably at least partly selected from hopeite, phosphophyllite and/or scholzite, particularly preferably at least partly selected from hopeite and/or phosphophyllite and very particularly preferably at least partly selected from hopeite. The maintenance of the phosphating quality in the acidic aqueous composition is therefore substantially based on the metered phosphates in particulate form which are contained in the activating aid (D). Without taking into account water of crystallization, hopeites stoichiometrically comprise Zn₃(PO₄)₂ and the nickel-containing and manganese-containing variants Zn₂Mn(PO₄)₃, Zn₂Ni(PO₄)₃, whereas phosphophyllite consists of Zn₂Fe(PO₄)₃, scholzite consists of Zn₂Ca(PO₄)₃ and hureaulite consists of Mn₃(PO₄)₂. The existence of the crystalline phases hopeite, phosphophyllite, scholzite and/or hureaulite in the activating aid (D) can be demonstrated by means of X-ray diffractometric methods (XRD) after separation of the particulate constituent (a) by means of ultrafiltration with a nominal cutoff limit of 10 kD (NMWC: nominal molecular weight cutoff), as described above, and drying of the retentate to constant mass at 105° C.

Due to the preference for the presence of phosphates comprising zinc ions and having a certain crystallinity, it is preferred for the formation of firmly adherent crystalline zinc phosphate coatings, in the process according to the invention, for the activating aid (D) to contain at least 20 wt. %, particularly preferably at least 30 wt. %, more particularly preferably at least 40 wt. %, of zinc in the inorganic particulate constituent, based on the phosphate content of the inorganic particulate constituent, calculated as PO₄.

However, the activating aid (D) should preferably not additionally contain any titanium phosphates, since these do not have a positive effect on the phosphating quality when metered in. In a preferred embodiment of the process according to the invention, the proportion of titanium in the inorganic particulate constituent of the activating aid (D) is therefore less than 0.01 wt. %, particularly preferably less than 0.001 wt. %, based on the activating aid (D). In a particularly preferred embodiment, the activating aid (D) contains a total of less than 10 mg/kg, particularly preferably less than 1 mg/kg of titanium.

The polymeric organic compound (a2) that stabilizes the particulate constituent has a major influence on the effectiveness of the particulate constituent (a) metered in via the activating aid (D). It has been shown that the selection of the polymeric organic compound is decisive for the extent of the activation of the metal surfaces in the acidic aqueous composition for zinc phosphating, which activation is known to be brought about by the dispersed polyvalent phosphates and which, as the present invention shows, surprisingly can also take place simultaneously with the layer formation.

In the context of the present invention, an organic compound is polymeric if its weight-average molar mass is greater than 500 g/mol. The molar mass is in this case determined using the molar mass distribution curve of a sample of the relevant reference value, which curve is established experimentally at 30° C. by means of size-exclusion chromatography using a concentration-dependent refractive index detector and calibrated against polyethylene glycol standards. The average molar masses are evaluated with the aid of a computer according to the strip method using a third-order calibration curve. Hydroxylated polymethacrylate is suitable as a column material, and an aqueous solution of 0.2 mol/L sodium chloride, 0.02 mol/L sodium hydroxide, 6.5 mmol/L ammonium hydroxide is suitable as an eluent.

It has been shown that the maintenance of the phosphating quality and thus activation of the metal surfaces in the zinc phosphating process step when brought into contact with the acidic aqueous composition is particularly successful, i.e. using relatively small amounts of active components of the activating aid (D), if the polymeric organic compound (a2) used to disperse the particulate inorganic compound (a1) is at least partly composed of styrene and/or an α-olefin having no more than 5 carbon atoms, the polymeric organic compound (a2) additionally having units of maleic acid, its anhydride and/or its imide, and preferably additionally having polyoxyalkylene units, particularly preferably polyoxyalkylene units, in the side chains thereof. Such polymeric organic compounds (a2) are therefore preferred according to the invention in the particulate constituent (a) of the activating aid.

The α-olefin in this case is preferably selected from ethene, 1-propene, 1-butene, isobutylene, 1-pentene, 2-methyl-but-1-ene and/or 3-methyl-but-1-ene and particularly preferably selected from isobutylene. It is clear to a person skilled in the art that the polymeric organic compounds (a2) contain these monomers as structural units in unsaturated form covalently linked to one another or to other structural units.

Suitable commercially available representatives of polymeric organic compounds (a2) are, for example, Dispex® CX 4320 (BASF SE), a maleic acid-isobutylene copolymer modified with polypropylene glycol, Tego® Dispers 752 W (Evonik Industries AG), a maleic acid-styrene copolymer modified with polyethylene glycol, or Edaplan® 490 (Münzing Chemie GmbH), a maleic acid-styrene copolymer modified with EO/PO and imidazole units.

In the context of the present invention, polymeric organic compounds (a2) that are composed at least partly of styrene are preferred.

The polymeric organic compounds (a2) used for colloidal stabilization of the particulate constituent (a) of the activating aid (D) preferably have polyoxyalkylene units that in turn are preferably composed of 1,2-ethanediol and/or 1,2-propanediol, particularly preferably of both 1,2-ethanediol and 1,2-propanediol, the proportion of 1,2-propanediols in the entirety of the polyoxyalkylene units being preferably at least 15 wt. %, but particularly preferably not exceeding 40 wt. %, based on the entirety of the polyoxyalkylene units. Furthermore, the polyoxyalkylene units are preferably contained in the side chains of the polymeric organic compounds (a2). A proportion of the polyoxyalkylene units in the entirety of the polymeric organic compounds (a2) of preferably at least 40 wt. %, particularly preferably at least 50 wt. %, but preferably no more than 70 wt. %, is advantageous for the dispersibility of said compounds.

For anchoring the polymeric organic compound (a2) with the inorganic particulate constituent (a1) of the activating aid, which is at least partly formed of polyvalent metal cations in the form of phosphates selected from hopeite, phosphophyllite, scholzite and/or hureaulite, and increased stability and ability of the particulate constituent (a) to activate in the acidic aqueous composition of the zinc phosphating, the organic polymeric compounds (a2) additionally have imidazole units, preferably such that the polyoxyalkylene units of the polymeric organic compounds (a2) are at least partly end-capped with an imidazole group, and therefore, in the preferred embodiment, terminal imidazole groups are present in the polyoxyalkylene side chain, the covalent linkage of the polyoxyalkylene units with the imidazole group preferably being carried out via a nitrogen atom of the heterocycle.

In a preferred embodiment, the amine value of the organic polymeric compounds (a2) is at least 25 mg KOH/g, particularly preferably at least 40 mg KOH/g, but preferably less than 125 mg KOH/g, particularly preferably less than 80 mg KOH/g, and therefore, in a preferred embodiment, the entirety of the polymeric organic compounds in the particulate constituent (a) of the activating aid also have these preferred amine values. The amine value is determined in each case by weighing out approximately 1 g of the relevant reference value—organic polymeric compounds (a2) or the entirety of the polymeric organic compounds in the particulate constituent (a)—in 100 mL of ethanol, titration being carried out using 0.1 N HCl titrant solution against the indicator bromophenol blue until the color changes to yellow at a temperature of the ethanolic solution of 20° C. The amount of HCl titrant solution used in milliliters multiplied by the factor 5.61 divided by the exact mass of the weight in grams corresponds to the amine value in milligrams KOH per gram of the relevant reference value.

It has therefore been proven to be advantageous for the polymeric organic compounds (a2), preferably also the entirety of the polymeric organic compounds in the particulate constituent (a), to have an acid number according to DGF CV 2 (06) (as of April 2018) of at least 25 mg KOH/g, but preferably of less than 100 mg KOH/g, particularly preferably of less than 70 mg KOH/g, to ensure a sufficient number of polyoxyalkylene units. It is also preferred for the polymeric organic compounds (a2), preferably also the entirety of the polymeric organic compounds in the particulate constituent (a), to have a hydroxyl number of less than 15 mg KOH/g, particularly preferably of less than 12 mg KOH/g, more particularly preferably of less than 10 mg KOH/g, determined according to method A of 01/2008:20503 from European Pharmacopoeia 9.0 in each case.

For a stable dispersion of the inorganic particulate constituents in the activating aid (D), it is sufficient for the proportion of the polymeric organic compounds (a2), preferably the entirety of the polymeric organic compounds in the particulate constituent (a), based on the particulate constituent (a), to be at least 3 wt. %, particularly preferably at least 6 wt. %, but preferably not exceeding 15 wt. %. The dispersed particulate constituent (a) of the activating aid (D) is the solids content that remains after drying the retentate of an ultrafiltration of a defined partial volume of the activating aid (D) with a nominal cutoff limit of 10 kD (NMWC: nominal molecular weight cutoff). The ultrafiltration is carried out by adding deionized water (κ<1 μScm⁻¹) until a conductivity of below 10 μScm⁻¹ is measured in the filtrate.

The activating aid (D) preferably contains no more than 40 wt. % of particulate constituent (a), based on the agent, since otherwise the stability of the dispersion and the technical handling behavior for continuous or discontinuous metering of the agent into the acidic aqueous composition of the zinc phosphating by means of metering pumps are no longer ensured or at least complex. This applies in particular with regard to the overall low amounts of particulate constituents (a) required to maintain the phosphating quality of a reference amount of the acidic aqueous composition for zinc phosphating. However, it is advantageous for the activating aid to be provided as a dispersion that is as stable as possible and at the same time as highly concentrated as possible. This can be achieved in particular when using the preferred polymeric organic compounds (a2) to disperse the particulate inorganic compound (a1), so that activating aids (D) are preferably used which contain at least 5 wt. %, but preferably no more than 30 wt. %, of particulate constituent (a) based on the agent.

In such concentrated aqueous dispersions of the activating aid (D), i.e. those having a proportion of 5 wt. % of particulate constituent (a) based on the agent, the activating aid (D) can, in the process according to the invention, additionally be characterized by its D50 value of more than 10 μm, which is correspondingly preferred. The agglomerates of the dispersed particles contained in the dispersion bring about the thixotropic flow properties that are favorable for the handling behavior of the activating aid (D). The tendency of the agglomerates to be highly viscous at low shear is favorable for their long shelf life, while the loss of viscosity when sheared makes them pumpable. Favorable flow properties are also obtained if the dispersion does not significantly exceed a D90 value of 150 μm; therefore, according to the invention, a D90 value of the aqueous dispersion of less than 150 μm, preferably less than 100 μm, in particular less than 80 μm, is preferred. In the context of the present invention, the D50 value or the D90 value denotes the particle diameter that is not exceeded by 50 vol. % or 90 vol. %, respectively, of the particulate constituents contained in the aqueous dispersion. According to ISO 13320:2009, the D50 value or D90 value can be determined from volume-weighted cumulative particle size distributions by means of scattered light analysis according to Mie theory immediately after dilution of the activating aid (D), in the form of the concentrated aqueous dispersion, to a dispersed particulate constituent of 0.05 wt. % with a corresponding amount of deionized water (κ<1 μScm⁻¹) at 20° C., using spherical particles and a refractive index of the scattering particles of n_(D)=1.52−i·0.1. The dilution is carried out in such a way that an amount of the concentrated dispersion corresponding to a volume of 200 mL of deionized water is added to the sample vessel of the LA-950 V2 particle size analyzer from the manufacturer Horiba Ltd., and is mechanically circulated there into the measuring chamber (setting of the circulating pump on the LA-950 V2: level 5=1167 rpm for a volume flow of 3.3 liters/minute). The particle size distribution is measured within 120 seconds after the dispersion has been added to the dilution volume.

The presence of a thickener can be advantageous for preventing the irreversible agglomeration of primary particles of the particulate constituent (a), in particular if the activating aid (D) is present as the concentrated dispersion described above. In a preferred embodiment of the process according to the invention, the activating aid (D) therefore contains a thickener, preferably in an amount which, in the shear rate range of from 0.001 to 0.25 reciprocal seconds, gives the activating aid (D) a maximum dynamic viscosity of at least 1000 Pas, but preferably below 5000 Pas, at a temperature of 25° C., and preferably leads to shear-thinning behavior, i.e. a decrease in viscosity with increasing shear rate, resulting at 25° C. at shear rates above those present at the maximum dynamic viscosity, so that the activating aid (D) has an overall thixotropic flow behavior. The viscosity over the specified shear rate range can in this case be determined by means of a cone and plate viscometer having a cone diameter of 35 mm and a gap width of 0.047 mm.

A thickener, within the meaning of the present invention, is a polymeric chemical compound or a defined mixture of chemical compounds which, as a 0.5 wt. % constituent in deionized water (κ<1 μScm⁻¹) at a temperature of 25° C., has a Brookfield viscosity of at least 100 mPa·s at a shear rate of 60 rpm (=rounds per minute) using a size 2 spindle. When determining this thickener property, the mixture should be mixed with water in such a way that the corresponding amount of the polymeric chemical compound is added to the water phase at 25° C. while stirring and the homogenized mixture is then freed of air bubbles in an ultrasonic bath and left to stand for 24 hours. The measurement value of the viscosity is then read within 5 seconds immediately after application of a shear rate of 60 rpm by the number 2 spindle.

The activating aid (D) preferably contains a total of at least 0.5 wt. %, but preferably no more than 4 wt. %, particularly preferably no more than 3 wt. %, of one or more thickeners, the total proportion of polymeric organic compounds in the non-particulate constituent of the aqueous dispersion preferably also not exceeding 4 wt. % (based on the dispersion). The non-particulate constituent is the solids content of the aqueous dispersion in the permeate of the above-described ultrafiltration after it has been dried to constant mass at 105° C., i.e. the solids content after the particulate constituent has been separated by means of ultrafiltration.

Certain classes of polymeric compounds are particularly suitable thickeners and are also readily commercially available. The thickener is thus preferably selected from polymeric organic compounds, which in turn are preferably selected from polysaccharides, cellulose derivatives, aminoplasts, polyvinyl alcohols, polyvinylpyrrolidones, polyurethanes and/or urea urethane resins, and particularly preferably from urea urethane resins, in particular urea urethane resins that are a mixture of polymeric compounds resulting from the reaction of a polyvalent isocyanate with a polyol and a mono- and/or diamine. In a preferred embodiment, the urea urethane resin results from a polyvalent isocyanate, preferably selected from 1,4-tetramethylene diisocyanate, 1,6-hexamethylene diisocyanate, 2,2(4),4-trimethyl-1,6-hexamethylene diisocyanate, 1,10-decamethylene diisocyanate, 1,4,-cyclohexylene diisocyanate, p-phenylene diisocyanate, m-phenylene diisocyanate, 2,6-toluene diisocyanate, 2,4-toluene diisocyanate and mixtures thereof, p- and m-xylylene diisocyanate, and 4-4′-diisocyanatodicyclohexylmethane, particularly preferably selected from 2,4-toluene diisocyanate and/or m-xylylene diisocyanate. In a particularly preferred embodiment, the urea urethane resin results from a polyol selected from polyoxyalkylene diols, particularly preferably from polyoxyethylene glycols, which in turn are preferably composed of at least 6, particularly preferably at least 8, more particularly preferably at least 10, but preferably less than 26, particularly preferably less than 23, oxyalkylene units.

Urea urethane resins that are particularly suitable and therefore preferred according to the invention can be obtained by first reacting a diisocyanate, for example toluene-2,4-diisocyanate, with a polyol, for example a polyethylene glycol, to form NCO-terminated urethane prepolymers, followed by further reaction with a primary monoamine and/or with a primary diamine, for example m-xylylenediamine. Urea urethane resins that have neither free nor blocked isocyanate groups are particularly preferred. As a constituent of the activating agent (D), such urea urethane resins promote the formation of loose agglomerates of primary particles, which are protected against further agglomeration and, when metered into the acidic aqueous composition for zinc phosphating, dissociate into primary particles. To further promote this property profile, urea urethane resins that have neither free or blocked isocyanate groups nor terminal amine groups are preferably used as the thickener. In a preferred embodiment, the thickener, which is a urea urethane resin, therefore has an amine value of less than 8 mg KOH/g, particularly preferably of less than 5 mg KOH/g, more particularly preferably of less than 2 mg KOH/g, determined according to the method as previously described for the organic polymeric compound (a2) in each case. Since the thickener is substantially dissolved in the aqueous phase of the activating aid and can thus be assigned to the non-particulate constituent of the aqueous dispersion, while component (a2) is substantially bound in the particulate constituent, an aqueous dispersion in which the entirety of the polymeric organic compounds in the non-particulate constituent preferably has an amine value of less than 16 mg KOH/g, particularly preferably of less than 10 mg KOH/g, more particularly preferably of less than 4 mg KOH/g, is preferred. It is further preferred for the urea urethane resin to have a hydroxyl number in the range of from 10 to 100 mg KOH/g, particularly preferably in the range of from 20 to 60 mg KOH/g, determined according to method A of 01/2008:20503 from European Pharmacopoeia 9.0. With regard to the molecular weight, a weight-average molar mass of the urea urethane resin in the range of from 1000 to 10000 g/mol, preferably in the range of from 2000 to 6000 g/mol, is advantageous according to the invention and therefore preferred, in each case determined experimentally, as previously described in connection with the definition according to the invention of a polymeric organic compound.

The activating aid (D) is an aqueous dispersion that preferably has a pH in the range of 6.5-8.0 and particularly preferably does not contain any pH-regulating, water-soluble compounds with a pK_(A) value of less than 6 or a pK_(B) value of less than 5.

The activating aid (D) can also contain auxiliaries, for example selected from preservatives, wetting agents and defoamers, which are contained in the amount necessary for the relevant function. The proportion of auxiliaries, particularly preferably of other compounds in the non-particulate constituent which are not thickeners, is preferably less than 1 wt. %.

The activating aid (D) can preferably be obtained as a concentrated aqueous dispersion by

-   i) providing a pigment paste by triturating 10 parts by mass of an     inorganic particulate compound (a1) with 0.5 to 2 parts by mass of     the polymeric organic compound (a2) in the presence of 4 to 7 parts     by mass of water and grinding until a D50 value of less than 1 μm     has been reached, as determined by means of dynamic light scattering     after dilution with water by a factor of 1000, for example by means     of Zetasizer® Nano ZS from Malvern Panalytical GmbH; -   ii) diluting the pigment paste with such an amount of water,     preferably deionized water (κ<1 μScm⁻¹) or service water, and a     thickener that a dispersed particulate constituent (a) of at least 5     wt. % and a maximum dynamic viscosity of at least 1000 Pas at a     temperature of 25° C. in the shear rate range of from 0.001 to 0.25     reciprocal seconds is set,     preferred embodiments of the activating aid (D) being obtained in an     analogous manner by selecting corresponding components (a1), (a2)     and the thickener in the amount that may be provided or required in     each case. Such a concentrated aqueous dispersion has excellent     stability and, due to its thixotropic flow behavior, also good     pumpability, so that the concentrated dispersion can be metered     directly into the zinc phosphating system tank in a controlled     manner.

With regard to the acidic aqueous composition for zinc phosphating, it is imperative for the formation of homogeneous, closed zinc phosphate layers that, in the process according to the invention, said composition contains at least

(A) 5-50 g/kg of phosphates dissolved in water, calculated as PO₄, (B) 0.3-3 g/kg of zinc ions, and (C) free fluoride, and a free acid in points greater than zero.

In this context, the amount of phosphate ions includes orthophosphoric acid and the anions, dissolved in water, of the salts of orthophosphoric acid, calculated as PO₄.

The proportion of the free acid in points in the acidic aqueous composition of the zinc phosphating of the process according to the invention is preferably at least 0.4, but preferably no more than 3.0, particularly preferably no more than 2.0. The proportion of free acid in points is determined by diluting a 10 mL sample volume of the acidic aqueous composition to 60 mL and titrating with 0.1 N sodium hydroxide solution to a pH of 3.6. The consumption of mL of sodium hydroxide solution indicates the point number of the free acid.

The preferred pH of the acidic aqueous composition is usually above 2.5, particularly preferably above 2.7, but preferably below 3.5, particularly preferably below 3.3. The “pH,” as used in the context of the present invention, corresponds to the negative common logarithm of the hydronium ion activity at 20° C. and can be determined by means of pH-sensitive glass electrodes.

A quantity of free fluoride or a source of free fluoride ions is essential for the layer-forming zinc phosphating process. Insofar as components comprising iron or aluminum surfaces, in addition to zinc surfaces, are to be zinc-phosphated in a layer-forming manner, as is necessary, for example, in the zinc phosphating of automobile bodies that are at least partly made of aluminum, it is advantageous if the amount of free fluoride in the acidic aqueous composition is at least 0.5 mmol/kg, particularly preferably at least 2 mmol/kg. The concentration of free fluoride should not exceed values above which the phosphate coatings have loose adhesions that can be easily wiped off, since this defect also frequently cannot be compensated for by increased metering of the activating aid (D) or by an increased steady-state amount of particulate constituents (a) in the acidic aqueous composition for zinc phosphating. Therefore, it is advantageous, and therefore preferred, for the concentration of free fluoride in the acidic aqueous composition of the zinc phosphating to be below 15 mmol/kg, particularly preferably below 10 mmol/kg and more particularly preferably below 8 mmol/kg, in the process according to the invention.

The amount of free fluoride can be determined potentiometrically by means of a fluoride-sensitive measuring electrode at 20° C. in the relevant acidic aqueous composition after calibration with fluoride-containing buffer solutions without pH buffering. Suitable sources of free fluoride ions are hydrofluoric acid and the water-soluble salts thereof, such as ammonium bifluoride and sodium fluoride, as well as complex fluorides of the elements Zr, Ti and/or Si, in particular complex fluorides of the element Si. In a phosphating process according to the present invention, the source of free fluoride is therefore preferably selected from hydrofluoric acid and the water-soluble salts thereof and/or complex fluorides of the elements Zr, Ti and/or Si. Salts of hydrofluoric acid are water-soluble within the meaning of the present invention if their solubility in deionized water (κ<1 μScm⁻¹) at 60° C. is at least 1 g/L, calculated as F.

In order to suppress what is known as “pin-holing” on the surfaces of the metal materials that are made of zinc, it is preferred, in such processes according to the invention, for the source of free fluoride to be at least partly selected from complex fluorides of the element Si, in particular from hexafluorosilicic acid and the salts thereof. The term pin-holing is understood by a person skilled in the art of phosphating to mean the phenomenon of local deposition of amorphous, white zinc phosphate in an otherwise crystalline phosphate layer on the treated zinc surfaces or on the treated galvanized or alloy-galvanized steel surfaces.

The accelerators known in the prior art can be added to the acidic aqueous composition in the process according to the invention for more rapid layer formation. These accelerators are preferably selected from 2-hydroxymethyl-2-nitro-1,3-propanediol, nitroguanidine, N-methylmorpholine-N-oxide, nitrite, hydroxylamine and/or hydrogen peroxide. It has been shown that comparatively less metering of activating aid is necessary or a smaller steady-state amount of particulate constituents (a) in the acidic aqueous composition for zinc phosphating has to be maintained if nitroguanidine or hydroxylamine is used as an accelerator, so that nitroguanidine or hydroxylamine, in particular nitroguanidine, are particularly preferred as accelerators in the acidic aqueous composition in the process according to the invention with regard to a particularly low substance consumption of the activating aid for maintaining the phosphating quality.

An embodiment in which less than 10 ppm of nickel and/or cobalt ions are contained in the acidic aqueous composition for zinc phosphating in the process according to the invention is particularly preferred from an ecological point of view.

Furthermore, in the process according to the invention, the additivation well known in the art in zinc phosphating processes can also be used.

In the process according to the invention, a good coating base for a subsequent dip coating or powder coating, in the course of which a substantially organic cover layer is applied, is produced. Accordingly, in a preferred embodiment of the process according to the invention, the zinc phosphating, with or without an intermediate rinsing and/or drying step, but preferably with a rinsing step and without a drying step, is followed by dip coating or powder coating, particularly preferably electrocoating, more particularly preferably cathodic electrocoating, which preferably contains water-soluble or water-dispersible salts of yttrium and/or bismuth in addition to the dispersed resin, which preferably comprises an amine-modified polyepoxide.

In a further aspect, the present invention relates to an acidic aqueous composition for zinc phosphating, which has a free acid in points greater than zero, and contains

-   (A) 5-50 g/kg of phosphates dissolved in water, calculated as PO₄, -   (B) 0.3-3 g/kg of zinc ions, -   (C) free fluoride, and -   (D) a water-dispersed particulate constituent comprising phosphates     of polyvalent metal cations, the phosphates at least partly being     selected from hopeite, phosphophyllite, scholzite and/or hureaulite,     obtainable by adding an amount of an aqueous dispersion to an acidic     aqueous composition containing the components (A)-(C),     the aqueous dispersion containing a particulate constituent (a) in     water-dispersed form, which constituent comprises     -   at least one particulate inorganic compound (a1) composed of         phosphates of polyvalent metal cations at least partly selected         from hopeite, phosphophyllite, scholzite and/or hureaulite,     -   and at least one polymeric organic compound (a2),         the aqueous dispersion being added in such an amount that the         proportion by weight of phosphates from the particulate         constituent of the aqueous dispersion, based on the acidic         aqueous composition containing components (A)-(C), is at least         0.0005 g/kg, preferably at least 0.001 g/kg, particularly         preferably at least 0.005 g/kg and very particularly preferably         at least 0.01 g/kg.

For the dispersed particulate constituent (a) as well as the at least one particulate inorganic compound (a1) or polymeric organic compound (a2), the same definitions and preferred specifications apply as those given above for the activating aid (D) of the process according to the invention. The same applies to the specification of the acidic aqueous composition for zinc phosphating, in particular with regard to components (A)-C), the free acid and further components such as those of the accelerator, which are each formed or selected in accordance with the acidic aqueous composition of the process according to the invention.

PRACTICAL EXAMPLE

To demonstrate the advantages of the process according to the invention, sheet portions of cold rolled steel (CRS), hot-dip galvanized steel (Z), and aluminum (AA6014) were zinc-phosphated. For this purpose, the sheets were:

-   a) alkaline-cleaned using 2 wt. % Bonderite® C-AK 1565 A mixed with     0.2 wt. % Bonderite® C-AD 1270 (each a commercially available     cleaning agent from Henkel AG & Co. KGaA) prepared in deionized     water (k<1 μScm⁻¹); cleaning was carried out by spraying after     setting a pH of 11.0 and a temperature of 55° C., initially for 1     minute at a pressure of 1 bar, then by immersion for 3 minutes while     stirring; -   b) rinsed with deionized water (k<1 μScm⁻¹) for approx. 1 minute; -   c) wetted with water, without further treatment in a separate     activation bath, by being directly immersed in a phosphating bath     based on deionized water (k<1 μScm⁻¹) and 4.6 wt. % Bonderite® M-Zn     1994 MU-1 and 1 wt. % Bonderite® M-AD 565 (each from Henkel AG & Co.     KGaA) and thus containing 1.3 g/L of zinc ions, 0.8 g/L Mn ions,     13.7 g/L PO₄, 1.0 g/L hydroxylamine, 0.9 points free acid and 27     points total acid for 3 min while stirring at 52° C., to which bath:     -   c1) no activating aid according to c2)-c3) was added, and which         was accordingly used as described in c); or     -   c2) 3 g Bonderite® M-AC AL3000 (Henkel AG & Co. KGaA) per liter         of the phosphating bath of an activating aid according to the         invention based on particulate zinc phosphate was added, which         activating aid, by means of a styrene-maleic acid copolymer that         additionally comprises polyoxyalkylene side chains, is present         in dispersed form, so that a proportion of particulate         constituents of the activating aid in the phosphating bath of         0.6 g per liter of the phosphating bath is realized; or     -   c3) 2 g Bonderite® M-AC 50 CF (Henkel AG & Co. KGaA) of a         Ti-based activating aid was added per liter of the phosphating         bath was added; -   d) then rinsed with deionized water (k<1 μScm⁻¹) for approx. 1     minute; -   e) blown with compressed air at 20° C. and then dried in an oven at     50° C.

Sheet portions of the abovementioned substrates were cleaned according to the sequence described above according to a) to b). Subsequently, the sheet portions were first phosphated according to c1), c2) or c3), and then rinsed, dried and weighed according to d) to e), before the created phosphate layer was then removed used a solution containing Bonderite® C-AK CW liquid at 25° C. and the substrates freed from the phosphate layer were in turn dried and weighed again. The phosphate layer weight produced in each case was determined from the resulting weight differences.

Table 1 gives the respective layer weights after going through the process steps and phosphating baths described above. It has been shown that in the process c2 according to the invention, with regard to layer weights and compactness of the zinc phosphate layers, equally good results are achieved on all substrates in the absence of activation upstream of the phosphating bath, whereas an activating aid based on particulate titanium phosphates is not suitable for carrying out phosphating under these conditions (see c3). The blind test c1 in turn shows that, in the absence of an upstream activation stage and without additivation of the phosphating bath with a suitable activating aid, there is no phosphating of the steel and aluminum surfaces, while very coarse phosphate crystallites grow on hot-dip galvanized steel (Z), so that a high layer weight, but no adequate corrosion protection can be achieved after dip coating.

TABLE 1 Layer weight/gm⁻² Visual assessment Test Z CRS Al Z CRS Al c1 6.2 N/A N/A ⊙

c2 3.7 2.6 2.2 + + + c3 N/A N/A N/A

Assessment scheme:

 no phosphate layer ⊙ non-conformal phosphating with coarse crystallites (maximum crystallite size from scanning electron microscope (SEM) images > 5 μm) + homogeneous, closed phosphating with fine crystallites (maximum crystallite size from SEM images < 5 μm) 

1. A process for anticorrosion pretreatment of a multitude of components in series, in which each component of the series at least partly has surfaces of the metals zinc, iron and/or aluminum and undergoes a zinc phosphating process step comprising contacting the surfaces of the metals with an acidic aqueous composition containing: (A) 5-50 g/kg of phosphates dissolved in water, calculated as PO₄, (B) 0.3-3 g/kg of zinc ions, and (C) free fluoride, and having a free acid in points greater than zero, wherein, in the zinc phosphating process step, an activating aid (D) is added continuously or discontinuously to the acidic aqueous composition in an amount that is sufficient, under selected conditions of the zinc phosphating process step, to maintain deposition by the acidic aqueous composition of a zinc phosphate layer having a layer weight of less than 5.5 g/m² on a hot-dip galvanized steel surface (Z), the activating aid (D) containing a particulate constituent (a) in water-dispersed form, which constituent comprises: at least one particulate inorganic compound (a1) composed of phosphates of polyvalent metal cations at least partly selected from hopeite, phosphophyllite, scholzite and/or hureaulite, and at least one polymeric organic compound (a2).
 2. The process according to claim 1, wherein the polymeric organic compound (a2) is at least partly composed of styrene and/or an α-olefin having no more than 5 carbon atoms, the polymeric organic compound (a2) additionally having units of maleic acid, its anhydride and/or its imide, and optionally additionally having polyoxyalkylene units in side chains of the polymeric organic compound (a2).
 3. The process according to claim 2, wherein the polymeric organic compound (a2) in the particulate constituent (a) of the activating agent (D) additionally has imidazole units.
 4. The process according to claim 2, wherein a portion of the polyoxyalkylene units present in the polymeric organic compounds (a2) are at least partly end-capped with an imidazole group.
 5. The process according to claim 1, wherein the proportion of polyoxyalkylene units in the polymeric organic compounds (a2) is at least 40 wt. % and does not exceed 70 wt. %.
 6. The process according to claim 1, wherein proportion of phosphates, calculated as PO₄, contained in the at least one particulate inorganic compound (a1), based on the dispersed inorganic particulate constituent (a1) of the activating agent (D), is at least 25 wt. %.
 7. The process according to claim 1, wherein the activating agent (D) further comprises at least one thickener (b) selected from urea urethane resins.
 8. The process according to claim 7, wherein the urea urethane resins of (b) have an amine value of less than 8 mg KOH/g.
 9. The process according to claim 1, wherein a total amount of the polymeric organic compounds (a2) in and based on the particulate constituent (a) of the activating agent (D) is at least 3 wt. %, but does not exceed 15 wt. %.
 10. The process according to claim 1, wherein the acidic aqueous composition for zinc phosphating has a pH below 3.6, and a free acid greater than 0.5 points.
 11. The process according to claim 1, wherein the acidic aqueous composition for zinc phosphating contains a source of free fluoride and at least 10 mg/kg, but no more than 200 mg/kg of free fluoride.
 12. The process according to claim 1, wherein the acidic aqueous composition contains an accelerator selected from 2-hydroxymethyl-2-nitro-1,3-propanediol, nitroguanidine, N-methylmorpholine-N-oxide, nitrite, hydroxylamine and hydrogen peroxide.
 13. The process according to claim 12, wherein the accelerator is nitroguanidine or hydroxylamine.
 14. The process according to claim 1, wherein, before contacting with the acidic aqueous composition in the zinc phosphating process step, the components of the series: (A) are not contacted with a colloidal, aqueous solution containing, in the particulate constituent, hopeite, phosphophyllite, scholzite and/or hureaulite; phosphates of polyvalent metal cations, or sparingly soluble salts of the element Ti; and/or (B) are not contacted with a colloidal aqueous solution for activating the surfaces of the particulate constituents for zinc phosphating; and/or (C) do not go through an activation stage for activating the surfaces of the components for zinc phosphating.
 15. The process according to claim 1, wherein, before contact with the acidic aqueous composition in the zinc phosphating process step, the components of the series are cleaned and optionally degreased in a cleaning stage.
 16. The process according to claim 15, wherein, the components are cleaned by contact in with an aqueous, alkaline cleaning agent, followed by the zinc phosphating process step, with or without an intermediate rinsing step, and the cleaning stage optionally does not include contact with an aqueous, alkaline cleaning agent containing a particulate constituent comprising hopeite, phosphophyllite, scholzite and/or hureaulite or sparingly soluble salts of the element Ti.
 17. The process according to claim 1, wherein the components of the series at least partly have hot-dip galvanized steel surfaces, and additionally have surfaces of the metals aluminum and iron.
 18. The process according to claim 13, wherein a zinc phosphate layer having a layer weight of at least 1.0 g/m², is deposited on the zinc surfaces.
 19. An acidic aqueous composition for zinc phosphating, which has a free acid in points greater than zero and contains (A) 5-50 g/kg of phosphates dissolved in water, calculated as PO₄, (B) 0.3-3 g/kg of zinc ions, (C) free fluoride, and (D) a water-dispersed particulate constituent comprising phosphates of polyvalent metal cations, the phosphates at least partly being selected from hopeite, phosphophyllite, scholzite and/or hureaulite, obtained by adding an amount of an aqueous dispersion to an acidic aqueous composition containing the components (A)-(C), the aqueous dispersion containing a particulate constituent (a) in water-dispersed form, comprising at least one particulate inorganic compound (a1) composed of phosphates of polyvalent metal cations at least partly selected from hopeite, phosphophyllite, scholzite and/or hureaulite, and at least one polymeric organic compound (a2), the aqueous dispersion being added in an amount such that proportion by weight of phosphates from the particulate constituent of the aqueous dispersion, based on the acidic aqueous composition containing components (A)-(C), is at least 0.0005 g/kg. 